The related characteristics of barometric pressure and wind speed help determine the destructiveness of a tropical cyclone -- the North Atlantic or Northeast Pacific version of which is commonly called a hurricane. These monster storms have the basic structure of a low-pressure center -- the “eye” -- encircled by howling winds and soaring thunderheads. The more extreme the cyclone’s pressure gradient, the fiercer its winds.
Barometric, or atmospheric, pressure is often simplistically defined as the weight at any given point of overlying air. More accurately, it’s proportional with the density of the gas molecules in a unit of air. In an area of low pressure -- and more widely spaced air molecules -- air tends to rise and become unstable, so low-pressure cells have the potential to be stormy, and even violent. In a hurricane, pressure is lowest in the eye and steadily mounts as you proceed outward through the eye wall -- that violent front of thunderstorms immediately girdling the eye itself -- and then through the rain bands composing the outer spirals. Barometric pressure is usually measured in millibars.
Wind is directly dependent on barometric pressure, as air flows from areas of high to low pressure. The spiral nature of a cyclone’s winds, which rotate counterclockwise around the low-pressure center, are due to the warping of this basic movement by the spin of the planet -- the Coriolis force -- and friction. The more pronounced the pressure gradient is, the swifter the winds are. In a hurricane, wind speed increases from the outer rain bands to the eye wall. There’s very little wind in the eye, which, subject to downdrafts, is often clear or only lightly veiled by high, wispy clouds.
Hurricanes spring from stormy cells called tropical disturbances, often triggered by easterly waves. The progression from tropical disturbance to full-blown tropical cyclone -- driven by the evaporation of warm ocean waters and the latent heat released as water vapors condense in rising air -- is marked in a series of stages ultimately defined by wind speed. A tropical depression evolves as a discrete low-pressure center and cyclonic winds become pronounced; if these winds exceed 17.5 meters per second (39 mph), a tropical storm is born. If the winds achieve 33 meters per second (74 mph), the storm officially becomes a hurricane. While the absolute value of barometric pressure is not a determining characteristic, most hurricanes have an eye below 990 millibars.
Both barometric pressure and wind speed are used to gauge the intensity of a given tropical cyclone. The most intense on record was Typhoon Tip, a mighty whirlwind that roared into Japan in the autumn of 1979. Typhoon Tip’s central pressure registered at 870 millibars on October 12 of that year. The storm also takes the prize for the largest cyclone yet measured. The immense typhoon had gale-force winds extending across a radius of 2,220 kilometers (1,380 miles). A 1996 storm named Tropical Cyclone Olivia, which made landfall in Australia, holds the current record for maximum sustained wind speed -- an astonishing 113 meters per second (253 mph).